Nitriding of chromium steel



United States Patent NITRIDING on CHROMIUM STEEL Teddy E. Hollingsworth, Robert C. Matter, and Oscar H. Smith, Jr., Anderson, Ind., assignors to General Motors Corporation, Detroit, Mich., a corporation of Delaware No Drawing. Application January 11, 1 254, Serial No. 403,422

2 Claims. (Cl. 148--16.6)

This invention relates to a heat treating method and is particularly directed to a method for heat treating finished parts made from chrome steels and the like.

In nitriding treatments of various metals, control of the extent of nitriding is normally rather simple. However, in the nitriding of chrome steels, this control is ditficult apparently due to an affinity for nitrogen which causes excessive brittleness when the penetration and concentration of nitrogen is too great. Similarly, in the heat treatment of chrome steels, it is useful to selectively control the extent of nitriding wherein substantially no nitrided surface is apparent to a point where the useful nitrided surface occurs without excessive brittleness. This invention is directed to such control wherein an atmosphere of hydrogen and nitrogen is used.

It is, therefore, the basic object of the invention to provide a method for the heat treatment of chrome steels wherein a protective atmosphere containing hydrogen and nitrogen is used and wherein an addition agent is used in connection with the atmosphere, which addition agent aid in the control of nitriding effects on the steel. In carrying out this object, it is a further object to utilize carbon monoxide in specific quantities in order to control the nitriding effect.

Further objects and advantages of the present invention will be apparent from the following description.

In heat treatment of chrome steels, for example;

AISI 410 AISI 416 AISI 420 AISI 430 AISI 440A or for that matter any of the usual chrome steels that are stainless in character, a useful atmosphere is one involving hydrogen and nitrogen. Such an atmosphere is relatively inexpensive and if properly controlled provides very desirable results.

However, in hydrogen and nitrogen atmospheres, small quantities of free ammonia are formed which dissociates and is detrimental to the treatment due to an apparent afiinity of the chrome steel for nascent nitrogen which causes excessive nitriding that creates undesired brittleness in the past. We have found that through the use of an addition agent, in controlled quantities to the atmosphere, it is possible to control the extent of nitriding 2,786,003 Patented Mar. 19, 1957 within predetermined and desired limits. Specifically, we propose to use an atmosphere having in the neighborhood of 10% hydrogen and nitrogen therein. This atmosphere is used within the heat treating furnace and is maintained at a temperature of 1750 F. to 1850 F. The chrome steel is brought to this temperature and soaked in the atmosphere for about one-half hour and is then cooled by circulating air around the retort so that the heat treated part is maintained in the protective atmosphere of hydrogen and nitrogen.

In order to control the extent of nitriding, we have found that additions of carbon monoxide to the atmosphere in specific amounts produce a very eflicient and accurate control. For example if four tenths of a cubic foot of carbon monoxide is introduced into 500 cubic feet of the aforementioned hydrogen-nitrogen atmosphere, no appreciable nitriding is apparent on chrome steels heat treated therein. When three tenths of a cubic foot of carbon monoxide is introduced, the extent of nitriding is quite small although it is apparent. Two tenths of a cubic foot produces a very useful nitrided surface while We have found that it is impossible to go below one tenth of a cubic foot addition without obtaining excessive brittleness. Thus additions of from one tenth to four tenths of a cubic foot per 500 feet of atmosphere may be used to control selectively the extent of nitriding of the chrome steel.

The gas atmosphere composed of hydrogen and nitrogen only is useful also in controlling surface grain boundary oxidation since there is no appreciable moisture present nor is there any formed since the atmosphere is substantially neutral.

While the embodiments of the present invention as herein disclosed, constitute preferred forms, it is to be understood that other forms might be adopted.

What is claimed is as follows:

1. In a method for nitriding chrome steel in a neutral atmosphere comprising in the neighborhood of about 10% hytrogen and 90% nitrogen at a temperature of 1750 F. to 1850 F., that step of adding to the atmosphere carbon monoxide in quantities of from .l to less than .4 of a cubic foot per 500 cubic feet of atmosphere whereby the extent of nitriding is controlled inversely as the quantity of carbon monoxide increases.

2. In a method for nitriding a chrome steel part in a neutral atmosphere comprising in the neighborhood of 10% hydrogen and 90% nitrogen and at a temperature of 1750 F., the steps of adding carbon monoxide to the said atmosphere in quantity of about .2 of a cubic foot per 500 cubic feet of atmosphere, maintaining the part in said atmosphere for a period and at said temperature of about one-half hour and finally slowly cooling the part in said atmosphere.

References Cited in the file of this patent UNITED STATES PATENTS 

1. IN A METHOD FOR NITRIDING CHROME STEEL IN A NEUTRAL ATMOSPHERE COMPRISING IN THE NEIGHBORHOOD OF ABOUT 10% HYTROGEN AND 90% NITROGEN AT A TEMPERATURE OF 1750* F.TO 1850* F., THAT STEP OF ADDING TO THE ATMOSPHERE CARBON MONOXIDE IN QUANTITIES OF FROM .1 TO LESS THAN .4 OF A CUBIC FOOT PER 500 CUBIC FEET OF ATMOSPHERE WHEREBY THE EXTENT OF NITRIDING IS CONTROLLED INVERSELY AS THE QUANTITY OF CARBON MONOXIDE INCREASES. 